Key principles that enhance success when restoring endodontically treated teeth


Authors_ Nadim Z. Baba, DMD, and Charles J. Goodacre, DDS, MSD
Restoring endodontica lly treated teeth and retain­ing them throughout life remains a challenge. Several factors play a key role in the long-term survival of en-dodontically treated teeth and associated restorations. The purpose of thisarticle is to identify the key principles that affect tooth and restoration survival.


Principle No. 1
Mostendodon tically trea ted posterior teeth should be restored with crowns to enhance their longevity




Clinicianshaveobservedadifferencebetweenendo-dontically treated teeth and vital teeth. Endodontically treated teeth fracture more often than vital teeth:They tend to break during extraction, and pulpless molars without crowns can fracture.1-7
Multiple studies have shown that endodontically treated teeth benefit from the placement of crowns.
Onestudydeterminedthatendodonticallytreated teeth without crowns were lost at six times the rate of those with crowns.5 Another study demonstrated that endo­dontically treated teeth without crowns were lost after an average time of 50 months, whereas endodontically treated teeth with crowns were lost after an average time of 87 months.'1
Fixed partial dentures have increased clinical failure when supported by endodontically treated abutment teeth compared with vital abutmentteeth.15"8However, while crowns significantly improved the success of endodontically treated posterior teeth, it has not been shown thattheyimprove the success of anterior teeth.9 Therefore, intact or minimally restored endodontically trea tedanterior teeth do notneed complete cove rageby a crown. They only need a crown when they are weak­ened by large and/or multiple coronal restorations or when they require significant color/form changes that cannotbemanagedbya more conservative treatment10
In contrast with the above studies, a group of researchers found similar success rates when they evaluated endodontically treated premolars restored with a post and direct composite resin restorations both with and without complete coverage. Similarly, a retrospective cohort study12 indicated that endodonti­cally treated molars thatareintact, except forthe access opening, could be restored successfully using compos­ite resin restorations After considering the available data, we recognize the potential benefits of using composite resin to restore posterior teeth that are intact except for a con­servative access opening. However, more clinical data is needed that identifies the long-term success of these teeth when occlusal wear and heavy forces or para-functional habits are present _
For this reason, we recommend thatendodontically treated teeth that have been previously restored receive crowns that encompass the cusps because of the oc­clusal forces that will be applied to cusps that have been weakened by previous tooth structure removal. Con­versely, it may be possible to avoid crownson some pre­viously restored posterior teeth with only conservative access openings and little to no wear visible that would indicate the presence of detrimental occlusal forces.
Another example of a tooth that may not need a crown is a mandibular first premolar, which typically has a small, poorly developed lingual cusp and a lack of occlusal interdigitation that might spread the cusps apartand induce fracture

Principle No. 2
Posts do not reinforce endodontically treated teeth. Their only purpose is to retain the 
core

Historically, the use of posts was based on the con­cept that they reinforce teeth. Virtually every laboratory study hasshownthateitherpostsdo not reinforce teeth or they decrease the fracture strength resistance of the tooth when a force is applied via a mechanical testing machine.14-" Additionally, studies have compared the fracture resistance of endodontically treated extracted teeth without posts or crowns with the fracture resist­ance of teeth restored with posts, cores and crowns
Maxillary incisors, without posts, resisted higher failure loads than the other groups with posts and crowns,'6 and mandibular incisors with intact natural crowns exhibited greater resistance to transverse loads than teeth with posts and cores' These studies show no evidence that posts have a strengthening reinforce-menteffect(Fig. 1).

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Fig. 1_ A radiograph of a fractured maxillary second premolar with a metallic prefabricated post.

Clinical studies have also failed to provide definitive support for the concept that posts strengthen endo­dontically treated teeth. When the radiographs of 200 consecutively treated patients were examined several years after endodontic treatment, it was determined that teeth with posts had significantly more apical periodontitis.2
An analysis of data from multiple clinical studies noted that 3 percent of teeth with posts fractured and found no evidence that posts enhanced the survival of teeth.26 Posts have had little enhancing effect on th clinical success of fixed partial denture abutments, b they have improved the clinical success of removabl partial denture abutments compared with endodonti cally treated abutments where no posts were used.10
Sinceclinical and laboratory data indicate that teeth are not strengthened by posts, their purpose is the retention of a core that will provide adequate reten tion and support for the definitive crown or prosthesis Unfortunately, this primary purpose has not been completely recognized. A survey demonstrated tha 24 percent of general dental practitioners felt tha posts strengthen the teeth.27 Another survey found that 62 percent of dentists over the age of 50 believed that posts reinforce the teeth (39 percent of part-time faculty, 41 percent of full-time faculty and 56 percen of non-faculty practitioners), whereas only 41 percen of dentists under the age of 41 did not believe this2 An additional survey found that 29 percent of genera dental practitioners felt that posts reinforced the tee and 17 percent of board-certified prosthodontists Sweden believed this too.2_
Since posts do not reinforce a tooth, they shoul only be used when the core cannot be retained by som other means

Principle No. 3
The radiographic minimal length of gutta-percha should be 5 mm to ensure an adequate apical seal

After the preparation of an endodontically treated tooth to receive a post the remaining gutta-percha at the apex is a barrier against the passage of bacteria to the peri-apical area.Several studies"-1 have found that there is greater leakage when only 2 to 3 mm of gutta­percha is present, but that the preservation of 4 to 5 mm of gutta-percha ensures an adequate seal? Although multiple studies have indicated that 4 mm producesan adequate seal.stopping precisely at 4 mm is difficult, and radiographic variations in angulation couldleadtoretentionoflessthan4mm.Therefore,5mm appears to be a safer minimal radiographic length than 4 mm (Fig. 2).


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Fig. 2_A radiograph of an excessively long parallel-walled post in a maxillary first premolar with a less than ideal amount of guttapercha remaining at the apex.


The best method of preserving the apical seal during preparation of a post space is use of the working length determined during endodontic treatment The same reference point on the tooth used during endodontic therapy should be used during the post preparation. Additionally, a canal preparation instrument with an appropriate diameter should be used in conjunction with a rubber stopper placed around the instrument at the proper location to help ensure that an adequate amountof gutta-percha is retained apically.
Three methods have been advocated for removal of gutta-percha during preparation of a post space without disturbing the apical seal: chemical, thermal and mechanical It has been determined that both hot hand instruments and rotary instruments can be safely used to remove condensed gutta-percha adequately when 5 mm is retained a pica I ly.29323-39
The immediate removal of gutta-percha after en­dodontic treatment has also been studied for its effect on the apica I sea I. Several studies have determined that the removal of gutta-percha immediately after root-canal treatment has no detrimental effect on the apical

Principle No. 4
The optimal post length for all teeth, except mo­lars, is determined by retaining 5 mm of apical gutta­percha and extending the post to the gutta-percha. For molars, only the primary root should be used and it should not extend more than 7 mm into that root.
Short posts should be avoided.

The appropriate length for a post should be based
on minimizing the potential for damage to the tooth, optimizing postretentionandmaintaininganappropri-ate apical seal for the root-canal filling. Several length guidelines have been proposed. A review of scien­tific data provides the basis for differentiating between these varied guidelines.
While short posts have never been advocated, stud­ies have shown that they are frequently observed on radiographs (Fig. 3).

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Fig. 3_A very short post in the root of a maxillary second premolar


It was found that only 34 percent of 327 posts were as long as the inciso-cervical length of the crown. An evaluation of 200 endodontically treated teeth determined that only 14 percent of posts were two-thirds or more of the root length." Another radiographic study of 217 posts determined that only 5 pe rcentof the posts were two-thirdstothree-fourthsof the root length.43 Rootfractures caused byhigh stresses occur more frequently when short postsareused,: whereas increasing the length of a post increases the root fracture resistance.
It was determined that posts that are three-fourths of the root length offered the greatest rigidity and pro­duced the least root deflection.21 However, use of this apparently optimal post length is difficult with many teeth. When a tooth has an average or below average rootlengthandthe post occupies two-thirds or moreof the rootlength, it is not possible to retain 5 mm of gutta­percha at the apex. Therefore, optimal post length is determined by retaining 5 mm of apical gutta-percha a nd extending the post to that depth.
The use of this post length guideline is appropriate for all teeth, except molars. A study of 150 extracted maxillary and mandibular molars determined that mo­lar post spaces should not be prepared more than 7 mm apical to the orifice of the root canal in the primary roots (the distal root of mandibular molars and the palatal root of maxillary molars) because of the increased likeli- hood of root perforation." Secondary roots (facial roots of maxillary molars and mesial roots of mandibular molars)cannotevenaccommodatepoststhatare7mm long without excess root thinning and the potential for perforation or rootf racture after restoration.Therefore, molar posts should notextendmorethan7mmintothe primary roots and secondary roots should be avoided whenever possible
.
Principle No. 5
Large diameter posts increase the possibility of root thinning, root perforation and root fracture. It is rec­ommended that posts not exceed one-third of the root diameter.

Increasing the diameter ofa postweakensthe remain­ing root It has been determined that stresses increase in a root as the post diameter increases; larger post diameters decrease the resistance to tooth fractured With la rge dia meter posts (1.5 mm or more), it was deter­mined''"'that there wasa six-fold increase in the potential for root fracture for every mm of increased postdia meter.
Studies have shown that root fracture is the second most common cause of post and core failure. Mul­tiple factors have been associated with the potential for root fracture, including large diameter posts, short posts, and threaded posts (fig. 4)

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Fig. 4_Athreaded post in a mandibular second premolar that caused a root fracture.

It is recommended that the post diameter not exceed one-third of the root diamete andthatthe postdia meter be proportionally related to average root dimensions.
To ensure that posts do not exceed one-third of the root diameter, the post diameter should be between 0.6 and1.2mm,dependingonthetooth a0nlypostprepa-rationinstrumentsthatmatchthedesi red dia meter of the postspaceshould be used. When using a particular brand of post make sure that the matching drill belongs to the same type of post (Tables 1 and 2).

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A good understanding of dental anatomy, the eon-figuration of the roots and their variations, and use of an appropriate instrument angulation help in avoiding root thinning and perforation. Instruments should be angled so that they follow the canal (Fig. 5).

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Fig. 5_Perfotation of the roots of a mandibular second molar, the result of post space preparation with instruments not held parallel to the root canals.


When posts are needed in premolars, they are best placed in the palatal root of the maxillary premolar and the straightest root of any mandibular premolar with multiple roots. Root taper, curvature and depressions should be reviewed prior to post preparation. When posts are needed in molars, they should be placed in roots that have the greatest dentine thickness.
These roots are known as the primary roots and they are the palatal roots of maxillary molars and the distal roots of mandibular molars. However, it is important to remember thatextension of a post more than 7 mm api­cal to the root-canal orifice in primary canals increases the risk of perforation." The mesial roots of mandibular molars and the facial roots of maxillary molars should be avoided if atall possible. Attention should alsobegivento avoidinginstrumentpressu re on the rootsurface towards the furcation, as this surface is thinned more easily than the outer surface owing to root curvature.
With all teeth, the apical 5 mm of the roots should be avoided because most rootcurvaturesoccurwithin 5mm oftherootapex^andentranceintothisarea increases the risk of excessive root thinning or perforation.

Principle No. 6
A cervical ferrule should engage 2 mm of tooth structure
to prevent root fracture

ferrules can be established by the core engaging tooth structure(coreferrule) or by the crownoverly-ing/encompassing sound tooth structure apical to the core (crown ferrule)." The data indicates that crown ferrules are more effective than core ferrules,717383 and crown ferrules increase the tooth's resistance to fracture.74 75601 n spite of the data supporting the benefit of crown ferrules, not all practitioners recognize their value. A survey published by Morgano etol. evalu­ated the percentage of respondents who felt a ferrule increased a tooth's resistance to fracture: 56 percent of general dentists, 67 percent of prosthodontists and 73 percentofboard-certifiedprosthodontistsfeltthatcore ferrules increased a tooth'sfracture resistance.
Different lengths and forms of the ferrule have been studied. The length and form are essential to the success of the "ferrule effect" When possible, encom­passing 2 mm of intact tooth structure around the en­tire circumference of a core createsan optimally effec­tive crown ferrule. Ferrule effectiveness isenhanced by grasping largeramountsof tooth structure. Theamount of tooth structure engaged by the overlying crown ap-pearstobemoreimportantthanthelengthofthepostin increasing a tooth's resistance to fracture (Fig. 6).
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Figs. 6a, 6b_Post and crown loosened from maxillary canine a few months after placement. Both the core/prefabricated post and the crown came off (a). Clinical photograph shows the absence of cervical tooth structure (ferrule) for retention of the crown (b|.

If insufficient cervical tooth structure remains to develop a ferrule, surgical crown lengthening or ortho­dontic extrusion should be considered to expose more tooth structure. In some situations, it may be prudent to extra eta tooth and replace itwithanimplantandcrown whenoneormoreofthefollowingconditionsispresent: a ferrule cannot be developed; crown lengthening would create an unacceptable esthetic environment or produce a furcation defect; or a short root is present that would not permit appropriate post length to be developed.


Principle No. 7
Until more long-term data is available, fiber-reinforced resin postsshouldbe used with caution.

For many years, the standard method of restoring endodontically treated teeth has been either a cus­tom cast post and core or a prefabricated metal post with a restorative material core/ A nationwide survey of dentists in 1994 reported that 40 percent of general practitioners used prefabricated posts, and the most popular post wasthe parallel-sided ser­rated metal post28The usage of prefabricated posts has undoubtedly increased even more substantially since the 1994 survey. The hig h dema nd for aesthetic restorations and all-ceramic crowns led to the de­velopment of a variety of non-metallic prefabricated post systems as alternatives to metal posts.
In addition to the esthetic advantages of non-metallic posts, laboratory studies have shown that the resin-based alternative posts have favorable physical and mechanical properties and there is less root fracture with fiber-reinforced resin posts than with metal posts. However, clinical studies of fiber-reinforced posts have produced a wide range of reported failure percentages, ranging from 0.0 percent after a mean of 2.3 years to 11.4 percent after two years9 Post loosening and root fracture have been the most commonly reported complications (Fig. 71.

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Fig. 7_A radiograph of afractured maxillary lateral incisor with a glass fiber post.

Because the core depends on the retentive capacity or tne post, the prognosisofthefinal restoration ishighlydependent on the retention of the post" Given the wide ra nge of reported failure percentages, it appears that addi­tional long-term clinical data is needed to determine the efficacy of fiber-reinforced posts.

Conclusion
Based on this review of available research, thefol-lowing clinical recommendations are made:
1. Crowns are not needed for intact or minimally restored anterior teeth except when substantia I color or form changes cannot be accomplished by more-conservative means.
2. Crowns should be placed on most endodonti-cally treated posterior teeth to enhance their long-term survival. There are some data that indicates posterior teeth that are intact, except for the access opening, can be satisfactorily restored with compos­ite resin ratherthan a crown. However, the long-term success of this more conservative treatment in the presence of heavy occl usa I forces is not known.
3. Posts weaken teeth and they should only be used when the core cannot be adequately retained by some other means.
4.Anadequateapicalsealisretained by preserving 5 mm of gutta-percha.
5. Short posts should be avoided, as they increase the potential for root fracture. For all teeth except molars, optimal post length is determined by retain­ing 5 mm of apical gutta-percha and extending the post to that depth. For molars, posts should only be placed in the primary roots (palatal roots or maxillary molars and distal roots of mandibular molars) and they should not be extended more than 7 mm apical to the orifice of the root canal owing to the possibil ity of root thinning or perforation.
6. The diameter of posts should not exceed one-third of the root diameter to minimize root thinning and the potential for root fracture. Post preparation instrument diameter should be matched to root diameters.
7. When crowns are placed on endodontically treated teeth, they shou Id encom pass 2 mm of tooth structure apical to the core whenever possible, since crown ferrules increase the resistance of teeth to fracture.
8. Until more long-term clinical data becomes available, fiber-reinforced resin posts should be used with caution owing to the wide range of reported failure rates in clinical studies.

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